In the recovery of permanent gases (i.e., gases with very low boiling points normally found in the vapor phase), such as argon, helium, oxygen and other atmospheric gases, it is desirable to adjust processing conditions to reduce substantially amounts of a contaminant gas and thereby improve production rates of the extraction product whether in liquid or gaseous form. In the recovery of argon from atmospheric air, for example, the concentration of the nitrogen component can vary over one or more orders of magnitude from tens to thousands of parts per million in response to relatively small changes in temperature, flow rate, pressure and other environmental plant conditions. In the past the nitrogen concentration has been controlled by monitoring temperature levels on certain trays and adjusting production rates of argon withdrawn from the auxiliary rectification tower, such as disclosed in U.S. Pat. No. 2,934,908 to Latimer, or by adjusting the reflux to the primary rectification unit, similarly in response to temperature levels, such as disclosed in U.S. Pat. No. 2,934,907 to Scofield. Adjustment to process conditions suffers from delays in response to sensed conditions inherent in the operation of the rectification process. More efficacious operation of the rectification process can be achieved by actual analysis of the nitrogen content in the process stream to be treated in the secondary rectification tower for maintaining the nitrogen content within a desired range, e.g., 20 to 2000 parts per million (ppm).
Qualitative and quantitative analyses of atomic or molecular species in the vapor phase by means of their absorption or emission spectra are well known in analytical chemical techniques. In atomic absorption spectroscopy, a beam of light is passed through a vapor containing the atomic species to be analyzed and the amount of the species present is determined by the amount of light absorbed by the vapor. In visible emission spectroscopy, the atomic species in the vapor phase are excited to emit light and the spectrum and intensity of the emitted light are analyzed to determine which species are present and the concentration of each. Various methods of exciting atomic species to emit radiation have been used, such as arcs, sparks, and flames. It is also known to excite the atomic species by contact with metastable atoms of an excited, relatively inert gas in a flowing gaseous medium.
In U.S. Pat. Nos. 3,951,607 and 3,996,010 issued to Robert B. Fraser on April 20 and Dec. 7, 1976, respectively, there is disclosed a gas analyzer for pulmonary uses where the gas to be analyzed is passed through an analyzing chamber including electrodes which are in contact with the gas for generating an emission atomic or molecular spectrum representative of the gaseous mixture which is sensed by detection devices provided with filters of diverse transmission characteristics wherein the information from each device is quantified (by computer) and the response displayed as indicative of the quantity of each component of the gas. Such analysis provides data for periods of time, but suffers from unanticipated spectral responses resulting from contamination, such as by oxidation of the internal electrodes.
There is a clear need for a method and apparatus for multicomponent gas analysis which is highly sensitive and of a large linear dynamic range and is capable of analyzing more than a single component thereof using relatively simple and reliable apparatus with minimal interference effects.